There are many types of apparatus which derive power from a pair of electric power lines, and which are substantially insulated from ground. Particularly, however, there is a class of vehicle which derives its power from overhead lines, where the chassis or body of the vehicle is insulated from ground because it runs on rubber tires or the like, and which may have extremely high exposure to the general public. Such vehicles may be trolley coaches or buses which have a pair of trolley poles, each of which contacts one of a pair of overhead lines from which the electric power to drive the motors of the coach or bus is derived. Other types of vehicles maybe certain subway trains which run on rubber tires, and certain types of mine equipment.
In such apparatus as a trolley coach, it is usual that the motor of the coach is well insulated, but because of their nature and the circuits in which they operate, the speed control devices which are manipulated by the operator or driver of the vehicle may have leakage resistance.
In a trolley system, there are usually just two trolley lines from which power is derived, and they are normally DC. However, the general circuits of this invention may be adapted for AC operation, as discussed in greater detail hereafter. In any event, at any one time and in any one section of line, one of the pair of trolley lines is substantially at ground potential or within very few volts of ground potential because of the current flow through that line and the finite resistance thereof. Nonetheless, for all purposes, one line might be considered to be a zero voltage (OV) line, and the other line might be considered to be a high voltage (HV) line. Trolley lines are not polarized -- that is, the left line is not always, say, the OV line and the right line the HV line -- so that the apparatus of this invention must be able to operate, regardless of which of the two trolley lines is the OV line and which is the HV line.
When leakage resistance occurs, it will appear between the chassis or body of the apparatus such as a trolley coach and the HV trolley line, no matter which is the HV line at any given instant in time, because of the circuit arrangements within the trolley coach. However, as the leakage resistance reduces, a higher current flows through it, and the voltage of the chassis or body of the trolley coach or other apparatus tends to float upwards away from ground potential, or the potential of the OV line. As noted, apparatus particularly such as trolley coaches may have very high exposure to the public, and it is important that there should be no chance of any person being shocked by coming into contact with the body or other portion of such apparatus while that person's body is at ground potential. Thus, certain limits are set for the voltage of the body or other portion of an apparatus which might be contacted by a human who is at ground potential, without hazard to that human; and of current which, when contacted at sufficiently high voltage by a human is made, would be hazardous to that human. The normal limits, particularly for DC operation, are 30 volts and 2 milliamps. The 30 volt limit is chosen because it, together with the few volts that the OV line might swing above ground potential are in total below the threshold voltage which a person having high sensivity might notice. The 2 ma. level is chosen because current above that level through the body of a person may be harmful to that person.
Thus, by interposing a known resistance between the body or other portion of the electric powered apparatus and the most negative of the lines from which it derives its power, in series with an incipient leakage resistance between the body or other portion of the apparatus and the most positive of the electric power lines, and by providing means for detecting the flow of current through the known resistance and for causing an alarm status to be indicated when the flow of current through that known resistance exceeds a predetermined value and thus when the voltage drop across the known resistance exceeds a predetermined amount, an apparatus is provided which will warn of leakage resistance which is or which may become hazardous. When an alarm status occurs but lasts for only a very short period of time, it may be helpful to inhibit an alarm signal for a short period of time so as to accommodate voltage swings due to start-up, or roll-through at junction insulators, etc. If a somewhat longer alarm status occurs, it may be helpful to provide one form of alarm indication; but if the alarm status lasts for a period of time such that real danger might exist, a different form of alarm signal might be given. Such alarm inhibition and delay circuits are provided by this invention.
Because of the circuit arrangement discussed in greater detail hereafter, the detector and alarm circuit provided by this invention may be adapted to AC operation as well as DC operation, for use in other types of operations such as mine or underwater electrically operated devices which are insulated from ground, but which derive their power from at least a pair of lines of which one is nominally or substantially at ground potential and where leakage resistance from the body of the apparatus to the higher voltage line may occur such that the potential on the body of the apparatus with respect to ground might rise to such a level as to be potentially hazardous.
Apparatus according to the present invention is usually adapted to be operated from an independent voltage source within the trolley coach or other apparatus in which incipient leakage resistance is intended to be detected. Such independent voltage sources usually comprise batteries of the lead-acid automotive type which are used for lighting and standby purposes, etc., and which are usually of quite low DC voltage.